(1) Field of the Invention:
The present invention relates to a magnetic developer having a self-friction-chargeability, and a process for the preparation thereof. More particularly, the present invention relates to a novel composite magnetic developer which can develop an elctrostatic latent image without any particular charging operation to form a developer image that can be transferred onto a plain paper of a low electric resistance without scattering of developer particles (broadening contours of the transferred image) and in which the developer composition can be kept constant at the developing and transferring steps, and a process for the preparation of this novel composite magnetic developer.
(2) Description of the Prior Art:
As the developer capable of developing an electrostatic latent image without using a particular carrier, there are known so-called one-component type magnetic developers comprising a fine powder of a magnetic material incorporated in developer particles.
As one type of such one-component magnetic developer, there is known a so-called conductive magnetic developer in which a find powder of a magnetic material is incorporated in developer particles to render the particles magnetically attractable and a conducting agent such as electrically conductive carbon black is distributed in the surfaces of the particles to impart an electric conductivity to the particles (see, for example, the specifications of U.S. Pat. No. 3,639,245 and U.S. Pat. No. 3,965,022). When this electrically conductive magnetic developer is caused to fall in contact with an electrostatic latent image carried by a supporting plate in the form of a magnetic brush to effect development, an excellent visible image free of so-called edge effects or fogs can be obtained. However, it is known that when an image of the developer is transferred onto an ordinary transfer sheet from the supporting plate, there arises a serious problem. More specifically, as described in Japanese Patent Application Laid-Open Specification No. 117435/75, when a transfer sheet having a resistivity lower than 3.times.10.sup.13 .OMEGA.-cm, such as plain paper, is used, it often happens that at the transferring step, developer particles are scattered to cause broadening of contours of the transferred images or reduction of the transfer efficiency. This undesirable tendency may be moderated to some extend by coating a toner-receiving surface of a transfer sheet with a resin, wax or oil having a high electric resistance, but this improving effect is reduced under high humidity conditions. Furthermore, the cost of transfer sheets is increased by coating of such resin or the like and the touch or feel of transfer sheets is degraded.
As another type of the one-component type magnetic developer, there is known a one-component type non-conductive magnetic developer consisting essentially of particles of a homogeneous intimate mixture of a finely devided magnetic material and an electricity-detecting binder. For example, the specification of U.S. Pat. No. 3,645,770 discloses an electrostatic photographic reproduction process in which a magnetic brush (layer) of the above-mentioned non-conductive magnetic developer is charged by corona discharge to a polarity reverse to the polarity of an electrostatic latent image to be developed, the charged developer is caused to fall in contact with the electrostatic latent image carried by a supporting plate to develop the electrostatic latent image and the so-formed developer image is transferred onto a transfer sheet. This electrostatic photographic reproduction process is advantageous in that a developed image can be transferred onto a so-called plain paper or a similar transfer sheet, but this process still involves some defects. For example, it is ordinarily difficult to uniformly charge the magnetic brush of the non-conductive magnetic developer and the deep portion of the magnetic brush is not sufficiently charged, and it also is difficult to form an image having a sufficiently high image density. Furthermore, since a corona discharge mechanism has to be disposed in the developing zone, the structure of the copying apparatus becomes complicated.
Recently, there have been proposed a process in which an electrostatic latent image is developed by utilizing charging of a non-conductive magnetic developer caused by friction between the developer and the surface of an electrostatic latent image-supporting plate (see Japanese Patent Application Laid-Open Specification No. 62638/75) and a process in which development is accomplished by utilizing dielectric polarization of a non-conductive magnetic developer (see Japanese Patent Application Laid-Open Specification No. 133026/76). These processes, however, are still insufficient.
In the former process, for example, the development conditions should be controlled very strictly. If the development conditions are not strictly controlled, fogging is readily caused in non-image areas (fogging is especially prominent when the degree of mutual contact between the surface of a photosensitive material and the top ends of spikes of toner particles is high), and such troubles as adhesion of the magnetic toner particles to a developing sleeve and blocking of the toner particles are caused. These troubles are especially serious when the copying operation is carried out continuously.
In the latter process, though fogging is not caused to occur, since a visible image is formed from an electrostatic latent image by a developing charge obtained by the dielectric polarizing effect induced in the magnetic toner, a latent image area of a low potential is not sufficiently developed. Accordingly, a low-density area of an original is not sufficiently reproduced in an obtained copy and a half-tone cannot be sufficiently reproduced according to this process.
Moreover, prints obtained according to these two process are poor in the image sharpness, and when a p-type photosensitive material such as selenium is used, images having a sufficiently high density can hardly be obtained according to these processes.
Magnetic developers hving a self-friction-chargeability have already been proposed. For example, Japanese Patent Application Laid-Open Specifications No. 35546/78 and No. 33152/78 disclose that a mixture of conductive magnetic toner particles with insulating non-magnetic toner particles or insulating magnetic toner particles is used as a developer. These composite magnetic developers have such a property that they are charged by friction among particles. Namely, they have a self-friction-chargeability. These composite magnetic developers are advantageous in that images of these developers can be transferred onto plain papers. However, in these developers, the composition suitable for frictional self-charging is changed while the developer is being actually used for formation of developer images, and therefore, in order to maintain a good balance in the developer composition, the composition should be adjusted during the use as in case of conventional two-component type developers.